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24Describing the mechanisms that drive variation in species interaction strengths is central 25 to understanding, predicting, and managing community dynamics. Multiple factors have 26 been linked to trophic interaction strength variation, including species densities, species 27 traits, and abiotic factors. Yet most empirical tests of the relative roles of multiple 28 mechanisms that drive variation have been limited to simplified experiments that may 29 diverge from the dynamics of natural food webs. Here, we used a field-based 30 observational approach to quantify the roles of prey density, predator density, predator-31 prey body-mass ratios, prey identity, and abiotic factors in driving variation in feeding 32 rates of reticulate sculpin (Cottus perplexus). We combined data on over 6,000 predator-33 prey observations with prey identification time functions to estimate 289 prey-specific 34 feeding rates at nine stream sites in Oregon. Feeding rates on 57 prey types showed an 35 approximately log-normal distribution, with few strong and many weak interactions. 36Model selection indicated that prey density, followed by prey identity, were the two most 37 important predictors of prey-specific sculpin feeding rates. Feeding rates showed a 38 positive, accelerating relationship with prey density that was inconsistent with predator 39 saturation predicted by current functional response models. Feeding rates also exhibited 40 four orders-of-magnitude in variation across prey taxonomic orders, with the lowest 41 feeding rates observed on prey with significant anti-predator defenses. Body-mass ratios 42were the third most important predictor variable, showing a hump-shaped relationship 43 with the highest feeding rates at intermediate ratios. Sculpin density was negatively 44 correlated with feeding rates, consistent with the presence of intraspecific predator 45 interference. Our results highlight how multiple co-occurring drivers shape trophic 46All rights reserved. No reuse allowed without permission.was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint (which . http://dx

Although parasites are increasingly recognized for their ecosystem roles, it is often assumed that free‐living organisms dominate animal biomass in most ecosystems and therefore provide the primary pathways for energy transfer. To examine the contributions of parasites to ecosystem energetics in freshwater streams, we quantified the standing biomass of trematodes and free‐living organisms at nine sites in three streams in western Oregon, USA. We then compared the rates of biomass flow from snails Juga plicifera into trematode parasites relative to aquatic vertebrate predators (sculpin, cutthroat trout and Pacific giant salamanders). The trematode parasite community had the fifth highest dry biomass density among stream organisms (0.40 g/m2) and exceeded the combined biomass of aquatic insects. Only host snails (3.88 g/m2), sculpin (1.11 g/m2), trout (0.73 g/m2) and crayfish (0.43 g/m2) had a greater biomass. The parasite ‘extended phenotype’, consisting of trematode plus castrated host biomass, exceeded the individual biomass of every taxonomic group other than snails. The substantial parasite biomass stemmed from the high snail density and infection prevalence, and the large proportional mass of infected hosts that consisted of trematode tissue (M = 31% per snail). Estimates of yearly biomass transfer from snails into trematodes were slightly higher than the combined estimate of snail biomass transfer into the three vertebrate predators. Pacific giant salamanders accounted for 90% of the snail biomass consumed by predators. These results demonstrate that trematode parasites play underappreciated roles in the ecosystem energetics of some freshwater streams.

understory algal cover during the day, but not during the night. Examining whether urchin mortality from predation is density dependent and how habitat complexity influences this relationship is imperative because behavioral changes and increases in urchin populations can have vast ecological and economic consequences in kelp forest communities.

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